Method for Heat-Treating a Component Which Consists of a Metal Material and Comprises at Least One Surface Section Coated with a Glaze or Enamel Coating

ABSTRACT

A method for heat-treating a component which consists of a metal alloy, in which or on which at least one surface section is coated with a glaze or enamel coating, includes heating the component to a heating temperature which at least equals a minimum quenching temperature, and quenching the component starting from a temperature which at least equals the minimum quenching temperature in order to produce a higher-strength microstructure in the component. The components can be heat-treated such that the glaze or enamel coating is reliably prevented from chipping. The glaze or enamel coating is pre-cooled to a pre-cooling temperature at least on its free surface prior to quenching, said pre-cooling temperature maximally corresponding to the temperature at which the glaze or enamel coating begins to soften, and wherein the cooling rate at which the glaze or enamel coating is cooled is lower than the cooling rate during quenching.

The invention relates to a method for heat-treating a component which consists of a metal alloy, in particular a light metal material, in which or on which at least one surface section is coated with a glaze or enamel coating.

As explained in detail in the article “Möglichkeiten und Grenzen der Emaillierung von Leichtmetallen” [The possibilities and limitations of enameling light metals] by Dr.-Ing. Wolfgang Kühn, published in Oberflächen Polysurfaces No 2/09, pages 6-9, enamel coatings are layers of glass which have been adapted to the substrates intended for them, above all in relation to the melting temperature and the thermal expansion coefficients. They combine the characteristics of a glass surface with the material and processing characteristics of metals. Unlike other coatings, during stoving of the respective enamel coating a glass-metal laminate is formed, in which between the glass material and the metal substrate intermediate layers form known as intermetallic phases. These ensure a particularly strong bonding of the coating with the metal. To this end, modern enamels are multicomponent mixtures, the eutectic of which is used at low stoving temperatures to achieve a very good mechanical hardness and chemical resistance.

It is also known from DE 10 2010 025 286 A1, that the internal surfaces of exhaust ducts of light metal cast parts, such as for example cylinder heads, for combustion engines can be effectively protected from thermal overstressing by at least in sections being coated with a coating which is formed of a glass material. The practical implementation of this proposal presents a particular challenge in that on the one hand the coating must withstand the mechanical and thermal stresses arising during operation and on the other must allow machining of sections of the respective component adjacent to the coated surface section without the risk of the coating chipping.

When the glaze or enamel coating described in WO 2015/018795 A1 is applied to the surfaces of components made from light metal, it has become apparent that such a coating can still safely withstand the thermal and mechanical stresses and reliably protect the light metal substrate, if the temperature, to which the component concerned is exposed during operation on the surface provided with the enamel coating, is much higher than the melting temperature of the light metal material and the coating itself. Such enamel powders are thus particularly suited to the coating of surfaces which are exposed during use to a hot exhaust gas flow. Such surfaces are typically present in the area of the exhaust gas-carrying ducts of components of combustion engines, cylinder heads, turbochargers and so on. Components of this kind are in practice generally produced by casting.

The content of WO 2015/018795 A1 is hereby included in the content of the present application by reference.

As explained in detail in WO 2015/018795 A1, a suitably composed slip or similar is applied to the surface section to be coated for the coating with the glaze or enamel coating. The component is then heated completely or at least in the area of the respective surface section to a stoving temperature. At this temperature, the glass matrix of the coating melts and a chemical bond is created between the coating and the base material of the component.

The mechanical properties of cast parts made from light metal materials, in particular aluminium alloys, can be precisely adjusted by suitable heat treatment. So, through solution annealing with subsequent quenching, during which the component is brought at high speed to a low target temperature, by way of example ambient temperature, the strength of the component can be considerably increased. It has proven particularly economical in so doing if the stoving and the heating to the quenching temperature, from which the quenching takes place, are carried out in one process.

Practical trials have shown, however, that chipping of the glaze or enamel coating can occur if following heating to a temperature above the normal stoving temperature of the glaze or enamel coating the quenching takes place at very high cooling rates, such as those which occur during water quenching. Nevertheless, quenching at precisely these high cooling rates frequently has to be used for components which are exposed during use to high mechanical stresses, such as for example cylinder heads of combustion engines. At the same time, these components are the typical application examples for components which are coated in areas of high thermal stress with a glaze or enamel coating of the type involved here.

Against this background, the problem has arisen of indicating a method with which it is possible, to heat treat components which consist of metal materials, in particular light metal materials and on which at least one surface section is provided with a glaze or an enamel coating, so that on the one hand maximum strength of the component is achieved and on the other chipping of the glaze or enamel coating is safely avoided.

The invention has solved this problem by the method indicated in Claim 1.

Advantageous configurations of the invention are indicated in the dependent claims and will be explained in detail below along with the general inventive concept.

With the method according to the invention for heat-treating a component which consists of a metal material, in particular a light metal material, in which or on which at least one surface section is coated with a glaze or enamel coating, the respective component, consistent with the state of the art set out above, is heated to a heating temperature, which at least equals a minimum quenching temperature. The component is then quenched, starting from a temperature which in turn at least equals to the minimum quenching temperature, in order to produce a higher-strength microstructure in the component.

The heating temperature, to which the component is heated prior to quenching, is calculated so that the temperature of the component at the start of the quenching process taking into account also possible temperature losses occurring due to transport of the component or other intermediate work stages, at least equals the minimum quenching temperature.

According to the invention, with such a method, the glaze or enamel coating prior to quenching at least on its free surface is now pre-cooled to a pre-cooling temperature, maximally corresponding to the temperature at which the glaze or enamel coating begins to soften.

Here, according to the invention, the cooling rate at which the glaze or enamel coating is pre-cooled, is lower than the cooling rate achieved during quenching.

Through the pre-cooling provided for according to the invention, therefore, the glaze or enamel coating is pre-cooled prior to quenching sufficiently slowly to a temperature which is lower than the component temperature, typically lower than the minimum quenching temperature, and at which the glaze or enamel coating hardens again. Here, the target temperature of the pre-cooling, within the meaning of the invention, is generally the temperature above which a softening of the glass matrix and the abovementioned chemical processes occur, as a result of which the glaze or enamel coating bonds permanently and firmly to the metal material of the component. In the course of the pre-cooling, the coating is cooled at least in the region of its free surface to a temperature below this target temperature.

Here, the method according to the invention has proven to be particularly suitable for the heat treatment of components, consisting of a light metal material, in particular an aluminium-based material.

The method according to the invention allows highly-stressable components to be manufactured, the glaze or enamel coating of which is fully maintained during quenching. Components with a glaze or enamel coating of component areas can undergo heat treatment with quenching without restrictions on the solution annealing temperature, even if the coating only allows considerably lower temperatures, in order to remain defect free. The maximum achievable mechanical properties of a cast material can therefore be used to their full extent.

Once the glaze or enamel coating has been softened, as a result of the heating, to the heating temperature corresponding at least to the minimum quenching temperature and thus above the typical stoving temperature set for stoving of the glaze or enamel coating, through the pre-cooling according to the invention it therefore hardens again at least to the degree that it firmly bonds to the metal substrate of the component and is thus able to withstand the rapid temperature change during subsequent quenching of the component as a whole, without chipping occurring.

Since the pre-cooling of the glaze or enamel coating is performed after heating of the component to the heating temperature, during pre-cooling it may be that the component similarly cools to a certain extent. In this case, therefore, the minimum quenching temperature, from which quenching of the component starts, lies lower than the heating temperature originally reached, or the heating temperature is set sufficiently high that the component temperature even after the drop in temperature occurring during pre-cooling lies above the minimum quenching temperature.

The pre-cooling temperature, to which the glaze or enamel coating is pre-cooled, can typically be at least 30° C., in particular at least 50° C. lower than the minimum quenching temperature.

With components made from an aluminium material, suitable stoving temperatures and thus the temperatures at which softening of the glass matrix of the glaze or enamel coating begins, are typically in the range of 480-650° C., in particular 510-540° C. Chipping of the glaze or enamel coating from components made from aluminium materials can therefore be prevented particularly reliably if the pre-cooling temperature is a maximum of 480° C., in particular a maximum of 470° C. or 450° C. In contrast, typical minimum quenching temperatures for components consisting of Al materials are at least 480° C., in particular at least 500° C., wherein quenching temperatures of at least 520° C., in particular at least 530° C., have in practice proven to be particularly advantageous.

According to the invention, the pre-cooling of the glaze or enamel coating to the pre-cooling temperature can be performed by passing a fluid flow across the glaze or enamel coating. A suitably temperature-controlled flow of gas is particularly suitable for this. Compressed air has proven to be a particularly advantageous cooling gas for this, since in the operational environment in which the method according to the invention is applied, it is readily available, and the compressed air flow can be easily adjusted so that it provides the required cooling. Obviously, however, other gases, such as a protective gas, by way of example nitrogen or similar, can be used if available or indicated by way of example to avoid reactions between the metal substrate and the gas flow. The respective gas flow can be directed by means of a nozzle device towards the glaze or enamel coating, to ensure cooling concentrated on the respective surface section coated with the glaze or enamel coating.

Physically, the layer thickness and the thermo-physical data result in a propagation speed of the temperature wave caused by the cooling medium, the progression of which is determined by what is known as the thermal conductivity or thermal diffusivity of the glaze or enamel coating. The metal substrate of the component remains unaffected provided the thermal wave does not reach the cast surface as a result of the pre-cooling. With the layer thicknesses of glaze or enamel coatings of the kind involved here normally used in practice, cooling times of a maximum of 60 seconds, in particular a maximum of 40 seconds, are generally sufficient for this. Cooling to below the necessary minimum quenching temperature for normal layer thicknesses can be reliably ensured by limiting the duration of the pre-cooling to a maximum of 20 seconds, in particular 5-20 seconds.

Typical layer thicknesses of the glaze or enamel coating are in the range of up to 5 mm, in particular up to 2 mm.

The specific duration of the pre-cooling of the glaze or enamel coating required in each case can be determined in the normal manner known to the person skilled in the art by experimental measurements on test pieces of the components to be heat-treated. To this end, on the one hand the drop in temperature of the glaze or enamel coating occurring during pre-cooling, and on the other the temperature profile in the area of the interface between the glaze or enamel coating and the metal material of the component supporting this, is recorded by metrological means or determined by theoretical means. Ideally, the duration of the pre-cooling is set so that the temperature of the light metal material of the component on the surface section which is coated with the glaze or enamel coating, at least equals the minimum quenching temperature.

It is essentially assumed here that for the purpose according to the invention it is sufficient if only the free surface of the glaze or enamel coating is cooled to the pre-cooling temperature, so that in the area of the glaze or enamel coating bordering the metal substrate a higher temperature of around the minimum quenching temperature, still prevails. Simply through a pre-cooling limited on the free surface of the coating and thus on the layers of glaze or enamel coating in the vicinity of this surface, the chipping of the glaze or enamel coating during subsequent quenching is prevented. Since the coating has at the same time already cooled on its surface, the unit so formed of component and layer base is under compressive stress, which also increases its resistance.

Practice-oriented cooling rates of the pre-cooling are in the range of less than 5 K/s, wherein for cooling rates of at least 0.5 K/s, the pre-cooling can take place quickly so that during pre-cooling of the glaze or enamel coating excessive cooling of the remainder of the component does not occur.

The heat treatment carried out according to the invention can normally be performed as solution annealing with subsequent quenching. If the components are cast components made from light metal materials, in particular Al materials, typical annealing times are 0.5-5 h.

The quenching of the component can then take place with cooling speeds of at least 5 K/s, in particular at least 7 K/s or at least 10 K/s. Cooling rates of up to 50 K/s have proven reliable in practice, wherein the specific cooling rates achieved in each case for components with highly variable wall thicknesses and localised material aggregations across the component volume can have a broad spread.

The actual quenching of the components can be carried out in the normal way after the pre-cooling of the glaze or enamel coating according to the invention. Water is a particularly suitable quenching medium here. However, if necessary, other quenching media can also be used such as spray mist, polymers, oils or gases.

The cooling rate achieved in each case can be set here in a similarly known manner by means of the temperature of the quenching medium. If water is used as the quenching medium then, by way of example, the water temperature can range up to boiling point, to avoid excessively high cooling rates in the component.

As already mentioned, the method according to the invention is particular suitable for the heat treatment of components for combustion engines, in which at least one duct is provided, at least one surface section of which is coated with the glaze or enamel coating.

To prevent excessive cooling of the sections of the component bordering the section coated with the glaze or enamel coating during pre-cooling, they can be shielded from the respective cooling medium by the application of screens, insulating materials and similar.

After quenching, the components that have been heat treated according to the invention can undergo other treatment steps, such as ageing, in a conventional manner, in order to further optimise their properties with regard to the respective application.

In the following, the invention is explained in more detail using a drawing showing an exemplary embodiment.

The single figure is a schematic representation of an example of a component of the kind to be heat treated according to the invention, in the form of a cylinder head 1 for a combustion engine in a section aligned transversally to the longitudinal extension of the cylinder head 1.

The cylinder head 1, cast from an aluminium casting material normally used for this purpose, by way of example an AlSi11- or AlSi10Cu0.5 Mg alloy, has a flat contact surface 2, by which during operation, by means of an if necessary interposed cylinder head gasket, not shown here, it rests on a similarly not shown engine block of the respective combustion engine. Here, the combustion engine has combustion chambers arranged in a row and pistons, similarly not shown here, that move up and down.

In the contact surface 2 spherical-shaped recesses 3 are formed in a quantity corresponding to the number of cylinders of the combustion engine, which in the stroke direction of the pistons of the combustion engine form the top edge of the combustion chambers of the combustion engine.

An inlet duct 5 fed in from the one longitudinal side 4′ of the cylinder head 1 opens into each of the recesses 3, via which during operation the respective fuel-air mixture is introduced into the combustion chamber. At the same time, an exhaust duct 6 leads from the respective recess 3, which is led to the opposing longitudinal side 4″ of the cylinder head 1 and via which the exhaust gas produced during the combustion process is taken away from the combustion chamber of the combustion engine.

The inner surfaces 7 of the exhaust duct 6 surrounding the exhaust duct 6 are during operation, in particular in the area around its intake port 8, subject to high thermal and mechanical stresses from the hot exhaust gas flowing into the exhaust duct 6 at high flow speed when the intake port is open.

To protect against these stresses, the inner surfaces 7 are coated with an enamel coating 9, the thickness of which is on average 400 μm and which covers the inner surfaces 7 over the entire length of the exhaust duct 6.

In a conventional manner, the cylinder head 1 has a quantity of exhaust ducts 6 corresponding to the number of combustion chambers and respectively associated valves, arranged one after the other in the longitudinal direction of the cylinder head 1 and each coated with an enamel coating 9.

The production and composition of the enamel coating 9 are described in the abovementioned WO 2015/018795 A1, the content of which has already been included in the present application.

To stove the enamel coating 9, the cylinder head 1 is first heated to a stoving temperature of 520° C., in order to stove the enamel coating 9 in such a way that it firmly bonds to the Al substrate of the component.

For the heat treatment according to the invention, the cylinder head 1 has been solution annealed for an annealing time of 1.5 hours at a heating temperature of 535° C.

At the end of the annealing time, the cylinder head 1 has been removed within 10 seconds from the annealing furnace, not shown here, and placed on a holding device 10.

The holding device 10 is part of a pre-cooling device, which also comprises a nozzle arrangement 11. The nozzle arrangement 11 has nozzles 12, each one of which is directed into the outlet port 13 of the respective exhaust duct 6 configured on the one longitudinal side 4″. Here, the surfaces of the longitudinal side 4″ surrounding the outlet port 13 are thermally screened from the environment by a screen 14 comprising a heat-resistant material.

The nozzle arrangement 11 is connected to a central compressed air supply 15, via which compressed air at ambient temperature reaches the nozzle arrangement 11. For pre-cooling the enamel coating 9, via nozzles 12 associated with the individual exhaust ducts 6 of the cylinder head, a flow of compressed air D is directed straight against the free surface 9′ of the enamel coating 9. Within 30-40 s, the enamel coating 9 is in this way pre-cooled to a pre-cooling temperature of less than 470° C.

The cylinder head 1 is then removed from the holding device 10 and immersed for 5 s in a water bath, the temperature of which is 95° C. In this way the cylinder head 1 is quenched to ambient temperature.

Artificial ageing at 200° C. for a period of 1-200 h can follow the quenching. In the example described here, an ageing period of 5 h was selected.

The measure of the hardening achieved by the heat treatment is the yield strength of the material from which the cylinder heads 1 are made.

Following the artificial ageing, cylinder heads 1 cast from AlSi10Cu0.5 Mg alloy, depending on the quenching medium used, for quenching starting from a quenching temperature in each case of 535° C., had the following yield strengths:

Quenching medium Yield strength Water 280 N/mm² Combi-Quench (Water|Air) 262 N/mm² Air shower 220 N/mm²

REFERENCE NUMERALS 1 Cylinder head (component) 2 Contact surface of the cylinder head 1 3 Recesses of the cylinder head 1 4′, 4″ Longitudinal sides of the cylinder head 1 5 Inlet duct 6 Exhaust duct 7 Internal surfaces of the exhaust duct 6 8 Intake port of the exhaust duct 6 9 Enamel coating  9′ Surface of the enamel coating 9 10  Holding device 11  Nozzle arrangement 12  Nozzles 13  Outlet port 14  Screen 15  Compressed air supply D Compressed air flow 

1. A method for heat-treating a component which consists of a metal alloy, in which or on which at least one surface section is coated with a glaze or enamel coating, comprising: heating the component to a heating temperature, which at least equals a minimum quenching temperature, and quenching the component starting from a temperature which at least equals the minimum quenching temperature, in order to produce a higher-strength microstructure in the component, wherein the glaze or enamel coating is pre-cooled to a pre-cooling temperature at least on its free surface prior to quenching, said pre-cooling temperature maximally corresponding to the temperature at which the glaze or enamel coating begins to soften, and wherein the cooling rate at which the glaze or enamel coating is pre-cooled, is lower than the cooling rate achieved during quenching.
 2. The method according to claim 1, wherein the glaze or enamel coating is pre-cooled to the pre-cooling temperature by passing a fluid across it.
 3. The method according to claim 2, wherein the fluid is a compressed air flow.
 4. The method according to claim 2, wherein the fluid is directed against the glaze or enamel coating by means of a nozzle device.
 5. The method according to claim 1, wherein the duration of the pre-cooling is set so that the temperature of the metal material of the component on the surface section which is coated with the glaze or enamel coating, at least equals the minimum quenching temperature.
 6. The method according to claim 1, wherein the cooling rate at which the glaze or enamel coating is pre-cooled, is lower than 5 K/s.
 7. The method according to claim 1, wherein the cooling rate, at which the glaze or enamel coating is pre-cooled, is at least 0.5 K/s.
 8. The method according to claim 1, wherein the cooling rate at which the component is quenched following pre-cooling of the glaze or enamel coating, is at least 5 K/s.
 9. The method according to claim 1, wherein the cooling rate at which the component is quenched following pre-cooling of the glaze or enamel coating, is a maximum of 50 K/s.
 10. The method according to claim 1, wherein the pre-cooling temperature is at least 30° C. lower than the minimum quenching temperature.
 11. The method according to claim 10, wherein the pre-cooling temperature is at least 50° C. lower than the minimum quenching temperature.
 12. The method according to claim 1, wherein the pre-cooling temperature is a maximum of 480° C.
 13. The method according to claim 1, wherein the metal material of the component is a light metal material.
 14. The method according to claim 1, wherein the component is a component for a combustion engine, in which at least one duct is provided, on which at least one surface section is coated with the glaze or enamel coating.
 15. The method according to claim 1, wherein the component is produced by casting.
 16. The method according to claim 13, wherein the metal material of the component is an aluminium-based material. 